Growing up in western Connecticut, fly-fishing for smallmouth bass and learning about the natural world through the field guides of Roger Tory Peterson, he knew by Grade 2 what he wanted to do. Like Darwin, he is drawn to a multitude of organisms, including mussels, clams, snails, worms and woodland salamanders, and has a Darwinesque obsession with whatever he chooses to study.
In the 1980s, Ms. Karstad says, her husband would stop the car at the sight of a cattail, and bring out his calipers to get an accurate measurement. He was studying a species that was moving west into the Prairies and hybridizing with one already there.
Dr. Schueler also is fascinated with the natural history of earthworms - the subject of Darwin's final book. The great man kept worms (which Dr. Schueler says aren't native to Canada but came with early settlers) in his study and performed all sorts of experiments, such as testing their hearing. (They're indifferent to shouting, he reported.) By this time, he had followed up The Origin of Species by tackling humanity's evolution and explaining his theory of sexual selection in 1871's The Descent of Man. The volume on worms appeared a decade later, the year before he died at 73, and was enormously popular.
Of course, Darwin didn't need a bestseller because he was independently wealthy, but Dr. Schueler is not. He is a research associate with the Canadian Museum of Nature in Ottawa, which is a non-paying position, and he and his artist wife are currently collaborating on a book about slugs and snails, finishing the work of a colleague who died after starting the project.
Whereas the Darwins had servants, Dr. Schueler and Ms. Kardstad wear extra clothes indoors to cut down on heating bills. But many of their pleasures are similar, and rooted in their love of nature. Just as the Darwins sometimes took a late-night walk to hear the nightingales, they spend their Friday evenings using a bright light to watch mud puppies frolic under the ice downstream from where the mill once stood.
Like Darwin, Dr. Schueler suffers from poor health. He is an insulin-dependent diabetic, a condition that needs careful management. Again like Darwin, he is an agnostic married to a devout Christian. They talk about this difference, but it isn't a source of tension.
"We agree to disagree," he says, although Ms. Karstad finds no conflict in believing in God and in evolution. "I don't feel threatened by it," she says.
THE HISTORIC DARWIN: CREDIT FOR UNLOCKING THE SECRETS OF LIFE
With the double anniversary, Darwin is bound to be much in evidence this year. And rightly so, according to experts such as Dalhousie's Brian Hall and W. Ford Doolittle, a biochemist at the university who studies the evolution of genes and genomes.
"Trying to make sense of biology without evolution is like trying to make sense of current affairs without history," Dr. Doolittle says, adding that natural selection and descent with modification "are what convinced scientists, and much of the public, that life, including our own, is not incomprehensible."
But for Dr. Schueler and Ms. Karstad, the grandeur of Darwin's vision is what is most worth celebrating.
He connected all living things - "endless forms most beautiful," as he described them in the final passage of The Origin of Species, with a common history.
"Darwin taught us we are related to everything," Dr. Schueler says.
That's why, before long, they will be defrosting the cattails, whose slender male flowers taste a bit like corn on the cob and have to be eaten in a similar fashion because of the tough filament that runs down the middle of each pollen-packed tube.
Also in the freezer is burdock, which looks like rhubarb until it starts to produce burrs. When boiled, roots from young plants turn from creamy to grey, but taste vaguely like parsnip. A run into Ottawa's Chinatown will provide sea cucumbers, which are sliced and fried, and a neighbour has brought over some venison.
Over the years, the phylum feast's menu has varied, depending on where the two have been living. In the 1980s, it featured meat from a minke whale hit and killed by a boat near British Columbia's Queen Charlotte Islands. To describe the taste, Dr. Shueler asks: "Have you ever had beaver? It is fishier than beaver."
Putting together the tribute meal would be a lot easier, he says, had Darwin been born in the summer. "We would have eaten earthworms."
Anne McIlroy is The Globe and Mail's science reporter.